Is There a Metallicity Ceiling to Form Carbon Stars? - A Novel Technique Reveals a Scarcity of C-Stars in the Inner M31 Disk

Abstract

Verbatim from NASA NTRS. Not paraphrased, not summarized.

We use medium-band near-infrared (NIR) Hubble Space Telescope WFC3 photometry with model NIR spectra of Asymptotic Giant Branch (AGB) stars to develop a new tool for efficiently distinguish- ing carbon-rich (C-type) AGB stars from oxygen-rich (M-type) AGB stars in galaxies at the edge of and outside the Local Group. We present the results of a test of this method on a region of the inner disk of M31, where we nd a surprising lack of C stars, contrary to the ndings of previous C star searches in other regions of M31. We nd only 1 candidate C star (plus up to 6 additional, less certain C stars candidates), resulting in an extremely low ratio of C to M stars (C=M = (3.3(sup +20)(sub - 0.1) x 10(sup -4)) that is 1-2 orders of magnitude lower than other C/M estimates in M31. The low C/M ratio is likely due to the high metallicity in this region which impedes stars from achieving C/O > 1 in their atmospheres. These observations provide stringent constraints to evolutionary models of metal-rich AGB stars and suggest that there is a metallicity threshold above which M stars are unable to make the transition to C stars, dramatically affecting AGB mass loss and dust production and, consequently, the observed global properties of metal-rich galaxies.

Authors

• Boyer, Martha L. Institute for Energy Analysis
• Girardi, L. Istituto di Astrofisica e Planetologia Spaziale (INAF-IAPS)
• Marigo, P. Padua Univ.
• Williams, B. F. Washington Univ.
• Aringer, B. Wien Univ.
• Nowotny, W. Wien Univ.
• Rosenfield, P. Washington Univ.
• Dorman, C. E. California Univ.
• Guhathakurta, P. California Univ.
• Dalcanton, J. J. Washington Univ.
• Melbourne, J. L. California Inst. of Tech.
• Olsen, K. A. G. National Optical Astronomy Observatories
• Weisz, D. R. Washington Univ.

Keywords

• Metallicity
• Ceiling
• Carbon